A parallel circuit having identical, known frequency changer circuits can be used to increase the maximally deliverable electric power to a load. Parallel partial frequency changers further can be operated with different gating angles. Due to the separate single or multiphase systems at the output of the partial frequency changers a resulting rotating field with a larger number of pulses is formed in the load by superposition of the rotating fields of the partial frequency changers caused in the load. Since more commutations of respective phaseshifted electrical output variables of the partial frequency changers per unit time therefore contribute to the formation of output variables of the complete frequency changer, these output variables have a spectrum of harmonics shifted toward higher frequencies, and therefore considerably less ripple.
In frequency changers or partial frequency changers with a dc circuit, such as an intermediate dc link internal to the circuit, chokes are used for impressing the current in the dc link. However, since the chokes are premagnetized by a dc field, they have only a qualified limiting effect on current rise it the dc current rises out of control. This can occur, for instance, in the case of a short circuit, since the saturation state is reached quickly.
In the operation of frequency changers constructed from parallel identical partial frequency changers, no appreciable deviations of the operating point should occur in parallel operation between the partial frequency changers. This is important since each of the partial frequency changers provides at the output a separate single or multiphase system which causes a rotating field of its own in the load. Thus, even during transients the instantaneous values of the dc currents in the dc circuits of the partial frequency changers themselves should deviate from each other only up to a maximally permissible value. In the event of sudden changes of the reference value or when there is a fault in one of the parallel partial frequency changers, as can occur when there is a short circuit due to a commutation failure caused by conduction-through of a valve in the inverter part of this partial frequency changer, undesirable operating point deviations between the parallel partial frequency changers could not be avoided in the prior art. Thus, there existed the danger that a heavily fanned-up transient short-circuit current cusp would occur in the dc circuit of the disturbed partial frequency changer, while the remaining parallel partial frequency changer would try to continue in normal operation at least until the for instance, a frequency converter constructed of several parallel partial frequency changers with intermediate dc circuits acts on a so-called "converter motor" for the operation of a synchronous machine, then an asymmetrically occurring short-circuit current in one of the parallel partial frequency changers will stress the rotating synchronous machine considerably. This leads to heavy control deviations in the controllers for the converters on the network and load sides in the individual partial frequency changer.
From Japanese Patent Publication JP No. 44-111 666 (A) in "Patent Abstracts of Japan", E-33, Nov. 12, 1980, Vol. 4/No. 162, a frequency converter is known in which three single-phase frequency changers with intermediate dc circuits and chokes in each of the respective intermediate links form a three-phase system for a load. The inverters at the output of the single-phase frequency converters are wired together metallically for forming the rotating field for the load. Furthermore, the chokes in the individual dc circuits are coupled to each other magnetically to increase the smoothing action. However, in this type of frequency changer, the coupling of the chokes does not provide any mutual precontrol of the dc current in the intermediate links of the respective singlephase frequency changers.